The present invention relates to respective methods for cutting a rare earth alloy, manufacturing rare earth alloy plates, and manufacturing rare earth magnets with a wire saw, and also relates to a voice coil motor including the rare earth magnet.
Conventionally, a technique of cutting an ingot of silicon with a wire saw to slice the ingot into a large number of wafers has been developed. Japanese Laid-Open Publication No. 6-8234, for example, discloses this technique. In accordance with this technique, a large number of wafers, each having a constant thickness, can be simultaneously sliced from an ingot by cutting the ingot with a multi-wire running while supplying some slurry containing abrasive grains thereto (free abrasive grain type wire saw technique).
Alternatively, in accordance with a known technique, an ingot of a rare earth alloy is sliced using a rotating slicing blade, for example. However, such a technique using a slicing blade results in an undesirably large cutting width, because the cutting edge of a slicing blade is thicker than that of a wire. Because of this inordinately large cutting width, such a technique is an inefficient use of valuable resources.
A rare earth alloy is suitable for use as a magnet material. A magnet has found a wide variety of applications and is now broadly used for various types of electronic appliances. Under the circumstances such as these, it is highly desirable to cut down on the manufacturing cost per rare earth magnet. And the cost would be considerably reduced if a great number of wafers could be produced simultaneously from an ingot of a rare earth alloy such that a cutting width can be reduced by the use of a wire saw and that each wafer has a constant thickness.
Voice coil motors, which are important applications of rare earth magnets, are being made in increasingly smaller sizes. Accordingly, the thickness of a rare earth magnet used for this application must be significantly reduced compared with that conventionally required. It would therefore be advantageous if a rare earth magnet used for a voice coil motor could be manufactured with a wire saw having a small cutting width.
However, no one has reported on successfully cutting a rare earth alloy in accordance with a practical wire-saw technique. The present inventors experimentally cut an ingot of a rare earth alloy with a wire saw of the free abrasive grain type. As a result, the present inventors found that since a slurry-circulating pipe was clogged up in a very short amount of time with fine powder and grinding debris (i.e., swarf or sludge) involved with wire sawing, no slurry could be supplied to the wire after that, and the wire eventually snapped. If the slurry was entirely replaced every several hours in order to avoid this problem, wire sawing had to be suspended while the slurry was being replaced. Thus, such machining is not suitable for mass production and it is virtually impossible to put such machining into practice.
The present inventors have also observed that since the sludge was easily deposited in a cut groove, the cutting resistance noticeably increased and the wire was even more likely to snap as a result. The sludge will be easily discharged from a cut groove if the viscosity of the slurry is reduced to a certain value. However, according to the experiments by the inventors, as the viscosity of the slurry was lower, abrasive grains in the slurry were less likely to reside on the wire, and as a result, the rigid, hard-to-cut rare earth alloy failed to be efficiently cut. It was found, therefore, that the viscosity of the slurry should be controlled.
Furthermore, the cutting accuracy was found considerably deteriorated because various unwanted operating failures frequently happened during the cutting process. For example, the wire often disengaged from rollers, around which the wire was wound, because the sludge was also likely to be deposited on the grooves of the rollers. None of these problems has ever been observed during cutting an ingot of silicon or glass in accordance with a conventional wire saw technique.
In general, a rare earth alloy includes a rigid tetragonal phase and a viscous rare earth rich phase. Cutting of such a rare earth alloy is difficult compared with cutting of silicon. Therefore, when a rare earth alloy is to be cut with a wire saw, a relatively large tension must be applied to the wire. If the wire tension is made large, however, a burden is placed on rollers for driving the wire, causing the rollers to abrade. In particular, the sludge of a rare earth alloy that tends to aggregate together may be deposited on the grooves of the rollers. This will further facilitate the abrasion of the rollers due to the wire tension.
If the wire tension is made small, the abrasion of the rollers is suppressed. However, the planarity (or flatness) of a cut face of work is reduced. This results in reduction in cutting accuracy, so that the resultant work is unsuitable for practical use.
The present inventors studied the possibility of producing the rollers from a metal material. It was found, however, that metal rollers were not usable because the wire slipped with respect to the metal rollers.
A prime object of the present invention is providing a method, as well as an apparatus, for cutting a rare earth alloy, capable of suppressing abrasion of rollers to ensure long-time continuous operation and improving the planarity of a cut face.
Another object of the present invention is providing a method for manufacturing rare earth magnets using the method for cutting a rare earth alloy.
Still another object of the present invention is providing a voice coil motor including a rare earth magnet manufactured by the method of the present invention.
The method for cutting a rare earth alloy of this invention includes cutting an object to be machined while supplying slurry containing dispersed abrasive grains between a wire and the object to be machined. The wire is driven with a drive member, at least a wire contact face of the drive member being composed of an organic polymer material including rubber and elastomer, and cutting is carried out while a tension in a range between 14.7 N and 39.2 N is applied to the wire.
Alternatively, the method for cutting a rare earth alloy of this invention includes cutting an object to be machined while supplying slurry containing dispersed abrasive grains between a wire and the object to be machined. The wire is driven with a drive member, at least a wire contact face of the drive member being composed of an organic polymer material, and a temperature of the slurry is controlled to fall within a predetermined range.
In a preferred embodiment, the method includes the steps of: collecting the slurry containing sludge produced during cutting of the object to be machined to remove the sludge from the slurry; and performing temperature control for the sludge-removed slurry.
In another preferred embodiment, the rare earth alloy is a Rxe2x80x94Fexe2x80x94B rare earth sintered magnet (where R is a rare earth element including Y).
Preferably, a viscosity of the slurry at 25xc2x0 C. is in a range from 92 to 175 mPaxc2x7sec.
Preferably, sludge is removed from the slurry with a magnetic separator. The magnetic separator preferably generates a magnetic field of 0.3 tesla or more in a region where the sludge is removed.
Preferably, the rare earth alloy is cut while the rare earth alloy is lowered from upward to downward with respect to the wire.
Preferably, the rare earth alloy is divided into a plurality of blocks and secured together, and at least part of the supply of the slurry is performed through gaps between the plurality of blocks.
In a preferred embodiment, means for supplying the slurry to the wire is disposed at a position upstream of a wire running direction with respect to the object to be machined.
In another preferred embodiment, the drive member is a roller made of ester-type urethane rubber.
The method for manufacturing rare earth alloy plates of this invention includes the steps of: producing an ingot of a rare earth alloy; and separating a plurality of rare earth alloy plates from the ingot by any of the above methods for cutting a rare earth alloy.
The method for manufacturing rare earth magnets of this invention includes the steps of: producing a rare earth magnet by compacting a rare earth alloy powder and sintering a compact; and separating a plurality of magnets from the rare earth magnet by any of the above methods for cutting a rare earth alloy.
The voice coil motor of this invention includes the rare earth magnet manufactured by the above method for manufacturing rare earth magnets.
In a preferred embodiment, the thickness of the rare earth magnet is in a range from 0.5 to 3.0 mm.
The apparatus for cutting a rare earth alloy of the present invention cuts an object to be machined while slurry containing dispersed abrasive grains is supplied between a wire and the object to be machined. The apparatus includes: means for supplying the slurry between the wire and the object to be machined; a drive member for driving the wire, at least a wire contact face of the drive member being composed of an organic polymer material; and means for applying to the wire a tension in a range between 14.7 N and 39.2 N.
Alternatively, the apparatus for cutting a rare earth alloy of this invention cuts an object to be machined while slurry containing dispersed abrasive grains is supplied between a wire and the object to be machined. The apparatus includes: means for supplying the slurry between the wire and the object to be machined; drive member for driving the wire, at least a wire contact face of the drive member being composed of an organic polymer material; a temperature detector for detecting a temperature of the slurry; and a cooling device for cooling the slurry to control the temperature of the slurry to fall within a predetermined range.
In a preferred embodiment, the apparatus further includes means for heating the slurry.
Preferably, the apparatus further includes a magnetic separator for separating sludge of the rare earth alloy produced during the cutting of the rare earth alloy from the slurry by use of a magnetic field.
Preferably, the magnetic separator generates a magnetic field of 0.3 tesla or more in a region where the sludge is removed.
Preferably, a viscosity of the slurry at 25xc2x0 C. is in a range from 92 to 175 mPaxc2x7sec.